Multilayer circuits containing base metal conductors interleaved with insulating dielectric layers are made from base metal and ceramic powders, both mixed with organic binders and other organic constituents. When fired, the organic constituents must be removed while the metal must be retained in its unoxidized state.
Multilayer base metal circuits are conventionally fired one layer at a time in a nitrogen atmosphere containing up to a few hundred ppm O.sub.2 to remove the organics. The metal oxide and not the metal in its elemental form if usually thermodynamically stable under these conditions. However, the rate of oxidation of the metal is usually low, especially in the relatively reducing environment creaated by the organic decomposition products. After the metal powder has sintered oxidation requires diffusion of the oxygen through the metal oxide layer to the bulk metal. Most of the O.sub.2 in the N.sub.2 atmosphere is added at lower temperatures where the diffusion rate through the metal oxide layer into the metal is relatively low. Sintering, then, partially protects the metal from oxidation when the next layer is fired on top of it.
However, a certain amount of oxidation is typical, especially when using organics that require high O.sub.2 concentrations in the firing atmosphere for removal. Firing in an O.sub.2 /N.sub.2 atmosphere often results in discoloration of the dielectric layer when over metal because of diffusion of metal ions and precipitation of metal atoms in the dielectric. The amount of metal oxidation and dielectric discoloration depend on the level of O.sub.2 used which in turn dependsd on both the quantity and nature of the organics present. In general, low levels of O.sub.2 give low metal oxidation but high dielectric discoloration, while high levels of O.sub.2 give low dielectric discoloration but high oxidation. Optimizing the O.sub.2 level under these circumstances is often difficult.
Cofiring, or simultaneous firing, of dielectric and metal layer in an O.sub.2 /N.sub.2 atmosphere is even more difficult than firing the layers one at a time because longer times are required to remove the organics and since the metal layers have not been previously sintered they have little protection against this oxidizing atmosphere.